The purpose of this proposal is to assess the degree of functional integrity and morphological specificity of sensory and motor nerves reinnervating muscle spindles. Two groups of cats will have the nerve to a muscle destroyed and allowed to regenerate into that muscle. Reinnervated spindles will be localized in the muscle by electrophysiologically recording from afferent units isolated in dorsal root filaments. In one group, spindles will be localized in the muscle, and prepared for light or electron microscopy. We will compare the morphology and pattern of regenerated sensory and motor terminals along the length of the spindle's nuclear bag (NB) and nuclear chain (NC) intrafusal (IF) fibers to that existing in normal spindles. The other group of cats will be used to exhaustively evaluate the ability of reinnervated spindles to signal phasic and static length changes of the muscle. To experimentally assess whether sensory or motor nerve fibers themselves are responsible for the localization pattern of terminals along IF fibers, two other groups of cats will be prepared. One group will have only sensory nerve fibers regenerating into the muscle; the other only motor nerve fibers reinnervating the muscle. Spindles in these cats will be biopsied and prepared for microscopic examination. By examining spindles at different stages of the reinnervation process, we hope to establish whether: 1) the previously reported lateness of fusimotor reinnervation is a function of regeneration rate for nerve fibers of different diameter or is dependent on a preexisting sensory innervation, 2) the morphology of regenerated nerve terminals alters over the course of the reinnervation process, 3) any localization pattern of reinnervated sensory or motor nerve terminals along the length of the IF muscle varies during the reinnervation process, 4) there is an initial preference of reinnervating sensory or motor terminals for NB or NC fibers, 5) functional reinnervation by sensory fibers is correlated with the conduction velocity (or diameter) of the nerve fiber, and 6) signalling of phasic or static length change in the muscle varies over the course of reinnervation.